Spica is a close binary star whose components orbit about each other every four days. They stay close enough together that they cannot be resolved as two stars through a telescope. The changes in the orbital motion of this pair results in a Doppler shift in the absorption lines of their respective spectra, making them a double-lined spectroscopic binary.[6] Initially, the orbital parameters for this system were inferred using spectroscopic measurements. Between 1966 and 1970, the Narrabri Stellar Intensity Interferometer was used to observe the pair and to directly measure the orbital characteristics and the angular diameter of the primary, which was found to be (0.90 ± 0.04) × 10−3 arcseconds, and the angular size of the semi-major axis of the orbit was found to be only slightly larger at (1.54 ± 0.05) × 10−3 arcseconds.[5]

The primary is classified as a Beta Cephei-typevariable star that varies in brightness over a 0.1738-day period. The spectrum shows a radial velocity variation with the same period, indicating that the surface of the star is regularly pulsating outward and then contracting. This star is rotating rapidly, with a rotational velocity of 199 km/s along the equator.[6]

The secondary member of this system is one of the few stars whose spectrum is affect by the Struve–Sahade effect. This is an anomalous change in the strength of the spectral lines over the course of an orbit, where the lines become weaker as the star is moving away from the observer.[13] It may be caused by a strong stellar wind from the primary scattering the light from secondary when it is receding.[14] This star is smaller than the primary, with about 7 times the mass of the Sun and 3.6 times the Sun's radius.[6] Its stellar classification is B2 V, making this a main-sequence star.[2]

Spica is a rotating ellipsoidal variable, which is a non-eclipsing close binary star system where the stars are mutually distorted through their gravitational interaction. This effect causes the apparent magnitude of the star system to vary by 0.03 over an interval that matches the orbital period. This slight dip in magnitude is barely noticeable visually.[15] Both stars rotate faster than their mutual orbital period. This lack of synchronization and the high ellipticity of their orbit may indicate that this is a young star system. Over time, the mutual tidal interaction of the pair may lead to rotational synchronization and orbit circularization.[16]

Spica is 2.05 degrees from the ecliptic and can be occulted by the Moon and sometimes by the planets. The last planetary occultation of Spica occurred when Venus passed in front of the star (as seen from Earth) on November 10, 1783. The next occultation will occur on September 2, 2197, when Venus again passes in front of Spica.[17] The Sun passes a little more than 2° north of Spica around October 16 every year, and the star's heliacal rising occurs about two weeks later. Every 8 years, Venus passes Spica around the time of the star's heliacal rising, as in 2009 when it passed 3.5° north of the star on November 3.[18]

A method of finding Spica is to follow the arc of the handle of the Big Dipper to Arcturus, and then continue on the same angular distance to Spica. This can be recalled by the mnemonic phrase, "arc to Arcturus and spike to Spica."

The name Spica derives from Latinspīca virginis "the virgin's ear of [wheat] grain". It was also anglicized as Virgin's Spike. Johann Bayer cited the name Arista. Another alternative name is Azimech, from Arabic السماك الأعزل al-simāk al-a‘zal 'the Undefended', and Alarph, Arabic for 'the Grape Gatherer'. Sumbalet (Sombalet, Sembalet and variants) is from an Arabic sunbulah "corn ear". [19]

^Moesgaard, Kristian P. (1973). "Copernican influence on Tycho Brahe". In Jerzy Dobrzycki. The reception of Copernicus' heliocentric theory: proceedings of a symposium organized by the Nicolas Copernicus Committee of the International Union of the History and Philosophy of Science. Toruń, Poland: Studia Copernicana, Springer. ISBN90-277-0311-6.